Apparatus for treating molten metal having a sealed treatment zone

ABSTRACT

An apparatus for treating molten metal includes a treatment vessel for holding molten metal and a removable cover or hood for the vessel. Sealing means are provided between the cover and the vessel to provide a gas tight treatment zone. The sealing means include mating peripheral flanges on the vessel and cover, with the cover flange having three spaced annular sealing strips for contacting the vessel flange. These sealing strips include a first outer annular strip formed of an elastomeric material to give an air tight seal between the cover and vessel and a second middle annular strip formed of a deformable fibrous refractory material to provide a thermal barrier between the interior of the vessel and the outer annular strip. A third inner annular strip is a rigid metal strip upon which the cover is supported on the vessel with the cover flange at a fixed distance above the vessel flange and the outer sealing strip and middle sealing strip compressed therebetween to a controlled degree.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of Provisional ApplicationSerial No. 60/326,328, filed Oct. 1, 2001.

FIELD OF THE INVENTION

[0002] This invention relates to an apparatus for the treatment ofmolten metal and, more particularly, for degassing molten metal in asealed treatment zone.

DESCRIPTION OF THE PRIOR ART

[0003] When many molten metals are used for casting and similarprocesses they must be subjected to a preliminary treatment to removeunwanted components that may adversely affect the physical or chemicalproperties of the resulting cast product. For example, molten aluminumand aluminum alloys derived from alumina reduction cells or metalholding furnaces usually contain dissolved hydrogen, solid non-metallicinclusions and various reactive elements. The dissolved hydrogen comesout of solution as the metal cools and forms unwanted porosity in theproduct. Non-metallic solid inclusions reduce metal cleanliness and thereactive elements and inclusions create unwanted metal characteristics.This treatment is frequently carried out in a vessel or trough sectionin-line with the metallurgical trough used to convey the metal from theholding furnace to the casting machine.

[0004] These undesirable components are normally removed from moltenmetals by introducing a gas below the metal surface by means of gasinjectors. As resultant gas bubbles rise through the mass of moltenmetal, the absorb gases dissolve in the metal and remove them from themelt. This process is often referred to as “metal de-gassing” or“in-line metal de-gassing”.

[0005] A typical prior gas treatment apparatus for molten metals isdescribed in Waite et al. U.S. Pat. No. 5,660,614, issued Aug. 26, 1997.This apparatus uses a hood to collect off-gases and dust, and variousbaffles are used to control the flow of metal through the de-gasser.Such a unit does not provide a gas-tight seal.

[0006] Molten metals are also treated by means of in-line filtration toremove particulates. Such filters generally consist of box-like ortrough-like arrangements containing a filter media which may be a porousrefractory plate or granular refractory material in a bed. Filter unitsare sometimes combined with de-gassers.

[0007] There are demands from environmental protection agencies toprovide a completely sealed unit for treatment of molten metals so thatless particulate is generated and less is exhausted to the atmosphere.The objective is to create a substantially air-free operation whichreduces dross and hence dust formation. This is particularly the casefor in-line metal degassing where there is a requirement to exhaust thetreatment gases from the treatment vessel without leakage into or out ofthe treatment vessel.

[0008] However, equipment for treatment of molten aluminum operates athigh temperatures and frequently agitates the molten metal, causingsplashing. At the same time operational considerations require periodicopening of the equipment, for example to skim off dross or otherwiseclean the equipment.

[0009] In English, U.S. Pat. No. 5,846,749, issued Dec. 8, 1998, a metalde-gassing apparatus is described which, along other things, attempts toprovide a gas-tight sealed treatment zone. Baffles or underflow weirsare provided at the inlet and outlet of the vessel to act as air-locks,and the patent shows a rather complicated system with a plurality ofbolts for holding a cover plate snuggly on the treatment vessel. Thisdoes not allow for the fact that the cover must be removed quite oftenfor servicing of gas injectors and for cleaning a residue dross.

[0010] In Sarlitto et al., U.S. Pat. No. 5,656,235, issued Aug. 12,1997, a metal de-gassing apparatus is described which includes air-locksat the inlet and outlet of the vessel. The cover appears in this case torest on the top surface of the vessel.

[0011] It is an object of the present invention to provide an improvedform of sealing arrangement between the cover plate of an apparatus fortreatment of molten metal and the treatment vessel.

[0012] It is a further object of the invention to provide a sealingarrangement which will provide an air-tight seal while permitting thetreatment vessel cover to be removed easily.

[0013] It is a further object of the invention to provide an in-linemetal treatment vessel that provides for minimum gas escape or airinflow whilst being readily opened for dross removal and other servicingrequirements.

SUMMARY OF THE INVENTION

[0014] The present invention in its broadest aspect relates to anapparatus for treating molten metal, preferably as an in-line treatmentvessel for treating molten metal flowing between a holding furnace andcasting machine. It comprises a treatment vessel for holding moltenmetal. A removable cover or hood is provided for the treatment vesselwith sealing means between the cover and the vessel to provide a gastight treatment zone. The sealing means comprises mating peripheralflanges on the vessel and cover. Lying between the flanges and adaptedto contact both flanges when the cover or hood is in place are threesealing strips. These three sealing strips include a first outer annularstrip formed of an elastomeric material for the purpose of giving an airtight seal between the cover flange and the vessel flange. A secondmiddle annular strip is formed of a deformable refractory material andthis is adapted to provide a thermal barrier between the interior of thevessel and the outer annular strip. A third inner metal annular strip isadapted to provide a mechanical protection for the middle annular strip.

[0015] In trying to obtain a good seal between a cover and a processingvessel, elastomeric materials are advantageous because of their sealingproperties. However, such materials are not generally resistant to hightemperatures and the reactive nature of metals being processed. Themetals being agitated in the vessel will occasionally splash and contacta seal. Seals having a modicum of leak tightness and good thermalresistance are sometimes formed from refractory rope, paper or similarmaterials, but these have poor resistance to molten aluminum or similarreactive metals, as well as lacking sufficient sealing properties toprovide an airtight seal.

[0016] In the three-part seal of the present invention the innermostsealing strip is a metal strip which serves to provide a first linemetal barrier preventing molten metal from penetrating further betweenthe flanges. It also preferably serves to support the flange of thecover plate at a fixed distance above the flange of the treatment vesseland thereby control the degree of compression on the other two sealingstrips. The middle sealing strip of deformable refractory material,preferably in the form of rope, felt or paper, is of such a size that itis squeezed somewhat between the cover plate and vessel flanges and thisprovides a partial seal as well as a thermal barrier. Such sealingstrips may be formed from an inner core of metal rope or mesh, or ofrefractory rope, which is then wrapped in a refractory cloth or strip.The outer sealing strip of elastomeric material provides a goodair-tight seal while being well protected by the middle and inner sealsfrom extreme temperatures and the destructive effect of the metals.

[0017] The inner metal sealing strip typically has a square orrectangular cross-section, while the middle sealing strip is typicallyof a round or oval configuration. The outer annular strip may be ofsquare or rectangular cross-section or of a rounded cross-section, e.g.oval or round.

[0018] All sealing strips are preferably annular sealing strips that arecontinuous around the sides and ends of the vessel and this isparticularly important for the outer elastomeric sealing strip whichprovides the airtight seal. The vessel cover may be round, square,rectangular, etc., and the annular sealing strips generally correspondto the shape of the cover.

[0019] In one embodiment of the invention, the flange face on thetreatment vessel is flat and the three annular sealing strips areconnected to the flange of the cover.

[0020] In another embodiment of the invention, the flange face on thetreatment vessel has one or more raised, curved ridges located so thatthat one or more of the three annular sealing strips will contact theridges. In particular it is advantageous to provide such a ridge for theouter elastomeric seal so as to improve the sealing capabilities.

[0021] The cover of the treatment vessel equipped with such three-partseals can be adapted so that in use it rests on the mating surface ofthe treatment vessel and seals under its own weight. If more positivesealing is required, quick release clamps may be used, but in eithercase, sealing is obtained without the use of bolts or similar devices,and the cover can be easily and quickly removed and replaced forservicing.

[0022] The treatment vessel of the invention is typically an apparatusfor de-gassing molten metal and it may include gas injector rotorsextending downwardly from the cover and into the treatment zone. It alsotypically includes ventilators for safely removing gases from thetreatment zone. The treatment vessel also preferably has baffles,airlocks, or underflow weirs positioned at the entrance and exit of thetreatment vessel to provide, along with the seal between the cover andvessel flanges, a completely air-tight and leak-tight vessel fortreatment of molten metal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a plan view in partial section of a de-gassing troughunit;

[0024]FIG. 2 is a vertical section along line A-A of FIG. 1;

[0025]FIG. 3 is a vertical cross-section of the trough unit;

[0026]FIG. 4 is a perspective view of the bottom of a cover;

[0027]FIG. 5 is a sectional view showing details of a sealingarrangement; and

[0028]FIG. 6 is a sectional view showing details of a further sealingarrangement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] The drawings show a preferred embodiment of the invention as itrelates to a de-gassing apparatus for molten metals. This includes ade-gassing trough unit 10 supported by a framework 11 and being lined bya ceramic liner 12. The trough holds a body of molten metal 13 which isfed in through molten metal inlet 14 and is discharged through moltenmetal outlet 15. Ceramic or refractory baffles 16 are provided at eachend of the treatment zone to act as “air-locks” at the point of metalentry and exit from the vessel. A flat metal flange 17 surrounds the topof the trough 10.

[0030] A cover unit 20 fits over the de-gassing trough unit 10 and sitson the flange plate 17. As seen from FIGS. 4 and 5, the cover unitincludes a heavy steel plate portion 21 which includes an edge flange22. The cover supports downwardly projecting gas injection rotors 26, ofwhich the shafts only are shown. Suitable rotors are described forexample in U.S. Pat. No. 5,660,614.

[0031] Fixed to the bottom face of the edge flange 22 are three sealingstrips. These sealing strips include a first inner seal 25 which is asolid steel strip of square or rectangular shape. This metal strip actsas a mechanical protection from metal splashing from the de-gassingtrough. The metal strip 25 also bears the weight of the cover 20 andallows a predicted amount of compression on seals 24 and 23. The middleseal 24 is made of a metal mesh or ceramic fiber rope wrapped inrefractory, e.g. silica, fabric material. Its principle characteristicis its ability to withstand high temperatures. The third sealing strip23 is preferably square or rectangular in shape and is formed of anelastomeric material, e.g. a silicon material, and acts as a barrieragainst gaseous emissions. It is held between shoulder portions 27. Thecharacteristics of this seal 23 are selected for its elasticity andflexibility, and thus ensures a tight seal on the surface of the trougheven if the surface is slightly irregular. In one particularly preferredembodiment, the inner seal is a solid steel strip having dimensions ofabout 22 mm by 22 mm, the middle seal 24 has a diameter of about 35 mmand the outer elastomeric seal has a width of about 35 mm and a heightof about 25 mm. When the cover 20 is in place on the de-gassing troughunit 10, it can be seen that the intermediate seal and the outer sealare compressed down to 22 mm which is the height of the steel strip 25.By supporting the cover on the inner steel strips 25, the controlledcompression of the middle sealing strip 24 and the outer sealing strip23 assure a longer seal life and also a better sealing efficiency.

[0032]FIG. 6 shows an alternate embodiment of the outer seal, in which acontinuous semi-circular ridge 30 is provided on the surface of thelower flange 17 so that it contacts and locally compresses theelastomeric seal 30, thus providing effective gas tight sealing.

[0033] The seal between the cover 20 and the de-gassing trough 10 ismaintained by the weight of the cover. However, quick release clamps(not shown) may also be used to hold the cover in place. When servicingof the de-gassing trough is required, the clamps are released and thecover is lifted by a lifting mechanism.

[0034] The de-gassing assembly also includes a ventilating system (notshown) for safely removing gases from the treatment zone.

1. An apparatus for in-line treatment of a molten metal, comprising: atreatment vessel for holding said molten metal, a removable cover forthe treatment vessel and sealing means between the cover and the vesselto provide a gas tight treatment zone, said sealing means comprisingmating peripheral flanges on said vessel and said cover, having betweensaid mating flanges three spaced annular sealing strips adapted tocontact said flat mating face, said three annular sealing stripsincluding an outer annular strip formed of an elastomeric materialadapted to provide an air-tight seal between the cover flange and vesselflange, a middle annular strip formed of a deformable refractorymaterial adapted to provide a thermal barrier between the cover flangeand the vessel flange and an inner metal annular strip providing amechanical protection for said middle annular strip.
 2. The apparatusaccording to claim 1 wherein the said vessel has an inlet and outlet forconnecting said vessel to a metallurgical trough, and wherein a baffleis placed at said inlet and outlets to act as an air-lock to prevent airentry or gas escape.
 3. The apparatus according to claim 1 wherein thethree annular strips are mounted on the face of one of the said matingflanges.
 4. The apparatus according to claim 1 wherein the inner metalsealing strip has a square or rectangular cross-section.
 5. Theapparatus according to claim 1 wherein the middle sealing stripcomprises a refractory rope, felt or paper.
 6. The apparatus accordingto claim 5 wherein the middle sealing strip comprises a ceramic fiberrope wrapped in refractory fabric.
 7. The apparatus according to claim 1wherein the middle sealing strip comprises a metal mesh core wrapped ina refractory fabric.
 8. The apparatus according to claim 4 wherein theouter annular strip has a square or rectangular cross-section.
 9. Theapparatus according to claim 1 wherein the outer annular strip has anoval or round cross-section.
 10. The apparatus according to claim 1wherein the three annular sealing strips are connected to the flange ofthe cover.
 11. The apparatus according to claim 10 wherein the coverrests on the treatment vessel with the inner metal sealing strip restingon the vessel flange and the middle and outer sealing strips slightlycompressed between the cover and vessel flanges.
 12. The apparatusaccording to claim 11 wherein the vessel flange has one or more curvedridges on its surface and one or more of the sealing strips contact thecurved ridges.
 13. The apparatus according to claim 12 wherein the oneor more curved ridges contact the outermost sealing strip.
 14. Theapparatus according to claim 1 wherein the treatment vessel is anapparatus for de-gassing molten metal.
 15. The apparatus according toclaim 14 which includes gas injection rotors extending downward from thecover and into the treatment zone.
 16. The apparatus according to claim1 wherein the cover rests on the treatment vessel under its own weight.17. The apparatus according to claim 1 wherein the cover is fixed to thetreatment vessel by way of quick release clamping means.
 18. Theapparatus according to claim 1 which includes ventilator means forremoving gases from the treatment zone.